A number of inks are known in the art for clichè ink transfer printing of color images on a contact lens. Examples of such inks include those disclosed in U.S. Pat. Nos. 4,668,240, 4,857,072, 5,272,010, and 5,414,477. Processes for making colored lenses using the above inks are substantially similar in that they all require (a) one or more pigments to add colorant to the contact lens; (b) one or more diluents to help the printing ink bind to the contact lens; (c) a binder polymer with appropriate functional groups; and (d) an adhesion promoter to form crosslinks with the functional groups of the binder polymer. The adhesion promoter can also chemically bond the binder polymer to the lens polymer.
Commonly assigned U.S. Patent Application publication No. 2003/0054109 to Quinn, et al. discloses an ink comprising at least one colorant, and a binder polymer having latent crosslinkable pendent groups (e.g., epoxy, hydroxy, alkenyl, isocyanate, peroxy, perester, anhydride, silane, and combinations thereof). Such inks are substantially free of a separate adhesion promoter species (e.g., hexamethylene diisocyanate or hexamethoxymethyl-melamine) and are typically used for hydrophilic contact lenses.
Inks including those disclosed in U.S. Pat. Nos. 4,668,240, 4,857,072, 5,272,010, and 5,414,477 and in U.S. Patent Application publication No. 2003/0054109 may have at least one of the following disadvantages. First, there may be adverse effects of a diluent in the ink mixture that could affect the ink and/or contact lens polymerization. However, a diluent in the ink mixture and subsequently in the contact lens may interfere with the required crosslinking of the binder polymer in the ink to the bulk contact lens. This may be an effect of diluting the concentration of reagents and/or the diluent may react with adhesion promoter to the detriment of the bonding needed to provide adequate adhesion of the colorant to the contact lens. In any case, the capability of the adhesion promoter to provide adequate adhesion of the colorant to the contact lens is diminished when using a poor diluent.
Second, inks described in the prior art may have issues of instability because of reactive functional groups present in adhesion promoter, activators (activating agent or agents), and binder polymers in the inks. Premature reactions may occur between the reactive functional groups, having a negative impact on print quality, ink pot life, and lens adhesion ability of the ink.
Third, inks described in the prior art may need to have an additional agent or a reactive coating to be added during the manufacturing process to promote adhesion of the color coat to a lens. This has all the disadvantages associated with adding an additional ingredient to a manufacturing process.
Fourth, inks described in the prior art may not provide a manufacturer flexbility for curing (gelling) inks or promoting lens adhesion of a color coat. For example, thermal curing is often performed after printing an ink on one of the molding surfaces of a plastic (e.g., polypropylene) disposable mold to react the adhesion promoter with functional groups of the binder polymer. Thermally curing of an ink on a plastic disposable mold prior to lens fabrication might distort the mold and result in a lens of unacceptable quality. In addition, thermal curing of an ink might result in thermal oxidative processes involving polypropylene mold surfaces. As such, thermal curing may negatively affect on cure kinetics and lens quality.
Commonly assigned U.S. Patent Application publication No. 2004/0044099 A1 to Tucker, et al. discloses a pad-transfer printing ink comprising at least one colorant, and a crosslinkable and water-soluble poly(vinyl alcohol). Although such ink is radiation-curable and suitable for producing a high-quality color image on a nelfilcon contact lens using a pad-printing system, it is impractical of producing a high quality color image on a hydroxylethylmethacrylate (HEMA)-based hydrogel contact lens (i.e., made of a copolymer of hydroxylethyl-methacrylate (HEMA) with other comonomers and/or crosslinkers) without an ink curing step and/or an adhesion promoter (e.g., a diisocyanate). For example, when applying directly to a HEMA-based contact lens, such ink (without an adhesion promoter) does not show adhesion, either after UV and/or thermal cure. Although an adhesion promoter can be added in an ink disclosed in U.S. Patent Application Publication No. 2004/0044099 A1 to provide a good adhesion of the ink to a HEMA-based hydrogel contact lens, premature reactions may occur between the reactive functional groups in the ink and as such, print quality, ink pot life, and lens adhesion ability of the ink may be adversely affected by the presence of the adhesion promotor.
Therefore, there still exist needs for inks and method for printing a high-quality color image on a hydrogel contact lens, in particular, on a HEMA-based hydrogel contact lenses.